Hierarchically self-assembled homochiral helical microtoroids.

Fabricating microscale helical structures from small molecules remains challenging due to the disfavoured torsion energy of twisted architectures and elusory chirality control at different hierarchical levels of assemblies. Here we report a combined solution-interface-directed assembly strategy for the formation of hierarchically self-assembled helical microtoroids with micrometre-scale lengths. A drop-evaporation assembly protocol on a solid substrate from pre-assembled intermediate colloids of enantiomeric binaphthalene bisurea compounds leads to microtoroids with preferred helicity, which depends on the molecular chirality of the starting enantiomers. Collective variable-temperature spectroscopic analyses, electron microscopy characterizations and theoretical simulations reveal a mechanism that simultaneously induces aggregation and cyclization to impart a favourable handedness to the final microtoroidal structures. We then use monodispersed luminescent helical toroids as chiral light-harvesting antenna and show excellent Förster resonance energy transfer ability to a co-hosted chiral acceptor dye, leading to unique circularly polarized luminescence. Our results shed light on the potential of the combined solution-interface-directed self-assembly approach in directing hierarchical chirality control and may advance the prospect of chiral superstructures at a higher length scale.